1. Enzymes: their Function and Uses
Enzymes are proteins, usually derived from living organisms, that are also catalysts for various metabolic or chemical reactions. Enzymes are therefore essential to all life. Recently, enzymes have been isolated, studied, altered, combined with other agents, and used in various processes. Uses of purified enzymes range from laundry detergents (where enzymes break down stains) to pathological detection of cancer (where enzymes produce a visible color product on tumor cells in a biopsy). Enzymes can be immobilized, for example by attaching the enzyme to a surface such as a bead, flat surface, or electrode using adsorption or covalent linkage. Immobilization allows the enzymes to be held in place for handling or to sustain washing without being removed. Immobilized enzymes can be used as biosensors, for example to measure glucose levels for diabetics.
As previously stated, enzymes are catalysts. As used herein a “catalyst” is defined as a material that increases the rate of a chemical reaction but is not itself consumed. At the end of a reaction, the catalyst is present in its original form so that it may act on new substrates. As used herein “substrate” is defined as a chemical that an enzyme works on to produce a new chemical. A “substrate” is the input material or “reactant” in the reaction catalyzed by the enzyme. Catalysts function by binding the substrate chemical or chemicals, and either introduce bond strain or orient reactants, thus making a transition or reaction possible at lower temperature or energy. Since enzymes are catalysts, they lower the activation energy barrier between two chemical states. Enzymes can, for example, facilitate the conversion of one chemical compound into another, or facilitate a reaction between chemicals. Without enzymes, reactions would be slow or, for most practical purposes, would not occur. This lowering of the activation energy barrier is one reason enzymes are required for living organisms. Enzymes control most body processes, and even cancer involves improper levels of certain enzymes regulating cell growth and death.
Enzymes fall into various classes relating to the type of reaction they catalyze, for example: oxido-reductases (such as dehydrogenases, oxidases); hydrolases (such as esterases, lipases, phosphatases, nucleases, carbohydrases, proteases); transferases; phosphorylases; decarboxylases; hydrases; and isomerases. Although enzymes within living cells act on specific compounds, it has been found that many enzymes will also act on other related compounds. Enzymes have also been found to perform similar reactions on synthetic or man-made substrates.
One use of enzymes is to perform reactions that convert a substrate into a detectable product. For example, a non-fluorescent compound may be converted into a fluorescent compound by cleavage of a particular bond using an enzyme. Alternatively, a colorless compound may be converted into a colored one by using an enzyme. Other uses of enzymes are deposition of a colored or otherwise detectable organic substrate from solution onto a solid support. This may be done by using an enzyme to make a soluble starting compound insoluble. Alternatively, enzymes can make a starting compound reactive, such as by forming a free radical thereof. The free radical subsequently reacts with, and binds to, the surrounding material. A useful embodiment of this technology is the ELISA test (Enzyme Linked ImmunoSorbant Assay), where, for example, an antigen is adsorbed to a solid support, such as a plastic microtiter plate well. To determine if an antibody to the antigen is present in a patient's serum, the serum is incubated in the coated well. If the antibody is there, it will bind to the immobilized antigen. After washing, a solution containing an anti-human antibody linked to the enzyme alkaline phosphatase is applied. The anti-human antibody will attach to any bound primary antibodies present. After washing, a substrate is applied, and if the alkaline phosphatase is present it will convert the colorless BCIP (5-bromo-4-chloro-3-indolyl phosphate) into a soluble color, which can then be measured spectrophotometrically. The amount of colored product produced is correlated with the amount of antibody in the serum, providing a quantitative measurement.
A number of significant advantages are gained by using enzymes for detection. These advantages include: a) Amplification: since the enzyme is a catalyst, and does not get used up in the reaction, and it can be used over and over. As more substrate is added more detectable product is produced. Except for practical limitations, the amount of product produced could be limitless. b) Linearity: the detectable product produced from the reaction of enzyme and substrate follows enzyme kinetics for that enzyme, and these can be relatively linear within some range. Even if the particular enzyme kinetics is not linear, the reaction may be calibrated. c) Selectivity: enzymes are usually very selective for the type of reaction and stereochemistry involved. Thus unwanted interferences may be reduced. d) Low background: if the conversion of the substrate to a colored or otherwise altered compound is negligible without the enzyme, then the background can be very low.
Enzymes themselves may be modified, for example, by genetic engineering or chemical modification, to produce alterations in specificity or reactivity, or to impart other characteristics, such as reduced immunogenicity (if the enzyme is to be used in vivo), or improved stability (for better shelf life or environmental tolerance).
A further expansion of the enzyme field relates to the use of unconventional material as biological catalysts, either proteins that are not normally enzymes, or non-protein material; for example, catalytic antibodies have been described.
2. Enzyme Substrates for Use in Detection Systems
The types of enzyme substrates popularly used for sensitive detection are typically calorimetric, radioactive, fluorescent or chemiluminescent. Conventional calorimetric substrates produce a new color (or change in spectral absorption) upon enzyme action. This type of detection is advantageous in that the colors produced are easily detected by eye or with spectral equipment. The cost of equipment for detection is also generally less than with other methods; for example in pathology, the brown color produced by the enzyme horseradish peroxidase acting on 3,3′-diaminobenzidine (DAB), requires only a simple bright field light microscope for observation of biopsied sections. A disadvantage of these colorimetric substrates is that they are generally of lower sensitivity than other enzyme methods.
Conventional radioactive substrates can enzymatically release or fix radioactivity for measurement. Although sensitive, this type of detection is becoming less popular due to the risks of handling and disposing of radioactive material, and since other methods now rival or exceed its sensitivity. Radioactive labeling for histochemical uses and autoradiography, typically require months to expose films, due to low specific activity, which is another disadvantage.
Conventional fluorescent substrates are popular since they are reasonably sensitive, generally have low backgrounds, and several differently colored fluorophores can be used simultaneously. A number of drawbacks, however, come with use of fluorescent substrates. Fluorescence requires expensive fluorescence optics, light sources and filters; by comparison, standard bright field microscopes are significantly less expensive. Fluorescence fades upon observation, sample storage or even exposure to room lights, thus making permanent or quantitative data difficult to achieve. Autofluorescence (fluorescence coming from certain compounds found naturally in many living organisms) from cells and other molecules can interfere with the test result. Standard tissue stains (such as nuclear fast red, hematoxylin, and eosin) cannot be seen simultaneously with the very different optics and illumination required for fluorescence detection, thus making visualization of landmarks of a tissue difficult. The standard viewing of tissues is done with bright field optics using colored stains and a standard microscope. Unfortunately, fluorescence is viewed using different illumination and sharp bandpass filters, so that only the fluorescent label is visible, and the general view of the stained tissue is not simultaneously available.
Chemiluminescence is based upon use of substrates that have sufficiently high chemical bond energies so that when the bonds are broken by an enzyme, energy is released in the form of visible light. This method has gained popularity due to the low background and very high sensitivity obtainable using photomultipliers, avalanche diodes or other sensitive light detectors. Alternatively photographic film can be used as a detection means. Chemiluminescence has a number of disadvantages. Detection requires expensive equipment or necessitates film development. The sample is not a permanent record, since the emitted light must be collected over time. Sensitive detection often requires lengthy light integration times of hours or even a day. Standard stains (such as nuclear fast red, hematoxylin, and eosin) cannot be seen simultaneously, thus making visualization of landmarks of a tissue difficult, for example. Only the emitted light from points in the specimen can be seen.
Additionally, all of the conventional detection schemes have some practical limitations for sensitivity. One limiting factor is the background, or non-specific signal generated. The background noise can come from various sources. For example with fluorescence detection the background can come from autofluorescence, fluorescent molecules that adhere to non-specific sites, light reflection off structures and other sources. Another limitation on detection method sensitivity is the amount of signal produced. For example, if an enzyme is used that has a low turnover, and produces only relatively few products, these few products will be harder to detect, and sensitivity will be worse than if a more efficient enzyme producing more products was used.
As mentioned earlier, since an enzyme is a catalyst and is not used up during reaction it can be fed more substrate, ideally forming product indefinitely. This provides a form of amplification. Of course there are practical limitations to enzyme amplification, such as the enzyme losing activity, the long times necessary to accumulate product, side reactions or other sources of background limit detection. Further, at some point the product produced may interfere with enzyme activity, either by shifting the reaction equilibrium, depositing products so as to impede flow to the active site of the enzyme or otherwise inhibiting the enzyme.
Although a number of enzyme based assays have been developed, one that is gaining popularity for sensitive detection is CAtalyzed Reporter Deposition (CARD), also known as Tyramide Signal Amplification (TSA, a trademark of New England Nuclear Corp, subsidiary of Perkin Elmer). In one variation of this method (there are several variations) a biotinylated antibody or nucleic acid probe detects the presence of a target by binding thereto. Next a streptavidin-peroxidase conjugate is added. The streptavidin binds to the biotin. Streptavidin is a protein isolated from the bacterium Streptomyces. Biotin is an organic compound having the formula C10H16N2O3S. A substrate of biotinylated tyramide (tyramine is 4-(2-aminoethyl)phenol) is used which presumably becomes a free radical when interacting with the peroxidase enzyme. The phenolic radical then reacts quickly with the surrounding material, thus depositing or fixing biotin in the vicinity. This process is repeated by providing more substrate (biotinylated tyramide) and building up more localized biotin. Finally, the “amplified” biotin deposit is detected with streptavidin attached to a fluorescent molecule. Alternatively, the amplified biotin deposit can be detected with avidin-peroxidase complex, that is then fed 3,3′-diaminobenzidine to produce a brown color. It has been found that tyramide attached to fluorescent molecules also serve as substrates for the enzyme, thus simplifying the procedure by eliminating steps. Although this type of assay has been used quite successfully, it has several drawbacks, including: expense of reagents, insufficient amplification, background problems, localization at the ultrastructural level (using electron microscopy) can be diffuse, and the limitations using fluorophores or chromophores mentioned previously.
3. Enzyme Biosensors
As used herein a biosensor is a device that uses biological materials to monitor the presence of a selected material, or materials, in a medium. Enzymes can be used in biosensor applications. Redox (reduction-oxidation) enzymes are used to generate an electrical signal, since electrons are transferred in redox reactions. Various other enzymes have been used in biosensors, and are selective for the following analytes, including use of beta-glucosidase to detect amygdalin, asparaginase for asparagine, cholesterol oxidase for cholesterol, chymotrypsin for esters, glucose oxidase for glucose, catalase for hydrogen peroxide, lipase for lipids, penicillinase for penicillin G, trypsin for peptides, amylase for starch, invertase for sucrose, urease for urea, and uricase for uric acid. Generally, biosensors achieve signal transduction using one of three approaches: amperometric, potentiometric and optical.
Amperometric biosensors work by enzymatically generating a current between two electrodes. The simplest design is based on the Clark oxygen electrode. The Clark oxygen electrode has a platinum cathode and a silver/silver chloride anode. Oxygen is reduced at the platinum cathode to water, and silver is oxidized to silver chloride at the anode. The rate of electrochemical reaction for the electrode is therefore dependent on the oxygen content of the solution. In a glucose monitor, glucose is a substrate for the immobilized glucose enzyme oxidase, which oxidizes glucose (consuming oxygen) to produce gluconic acid and hydrogen peroxide. This change in oxygen content alters the electrode current.
A variation on the above method measures the hydrogen peroxide produced by the enzymatic oxidation of glucose by making platinum the anode, and biasing it to 0.7 volts such that the hydrogen peroxide is oxidized back to oxygen, producing 2 electrons. Although the glucose oxidase is selective for glucose, and does not react with the closely related sugar fructose, some other molecules frequently found in the blood, either products of normal metabolism (e.g. uric acid) or drugs/medicaments taken orally (e.g. paracetamol or Vitamin C), can also break down directly and electrochemically at the electrode, bypassing the enzyme and giving a spurious signal. Similarly, the enzyme/device interface in other types of known biosensors is often prone to similar non-specific signals. In another variation redox enzymes may be coupled to other enzymes that interact with a specific substrate of interest to produce a product, the product then driving the redox enzyme.
Potentiometric biosensors are usually based on ion-selective electrodes. Such devices measure the release or consumption of ions during a reaction; the simplest potentiometric biosensor is based on a pH-probe. Glucose oxidase, for example, catalyzes the oxidation of glucose to gluconate, producing H+ ions and hydrogen peroxide. The H+ ions are then sensed by the pH probe. Detection is usually in the 10−4 to 10−2 M region, and therefore the above method generally lacks the accuracy and precision required for many analytes.
Optical biosensors have two common designs. In a first design light absorption is measured. An example is light absorption through a dye having a changed color that is the result of an enzyme driven pH change. A second design is based on measuring luminescence. An example is use of the enzyme firefly luciferase that reacts with ATP (adenosine triphosphate) and oxygen to produce AMP (adenosine monophosphate), PPi (inorganic pyrophosphate), oxyluciferin, CO—2 and a photon of light. This reaction can be coupled to any enzyme that produces or consumes ATP.
4. Metals and Enzymes
Some enzymes contain essential metal ions that are bound and required for activity. The metal ions aid in constraining the substrate for the reaction, but are not themselves consumed or deposited, they are part of the catalyst. Examples of enzymes that contain or require metal ions as cofactors are: alcohol dehydrogenase, carbonic anhydrase, and carboxypeptidase, which all require zinc ions; some phosphohydrolases and phosphotransferases require magnesium ions, arginase requires manganese ions; cytochromes, peroxidase, catalase, and ferredoxin contain iron ions; tyrosinase and cytochrome oxidase contain copper ions, pyruvate phosphokinase requires potassium ions, and plasma membrane ATPase requires sodium ions. However, these metal ions do not serve as substrates, are not linked to substrates and do not deposit as metal.
A very few metal ions are known to interact with enzyme reaction products. For example, the enzyme horseradish peroxidase can produce a diaminobenzidine (DAB) polymer. Nickel or cobalt ions complex with the DAB polymer to give the polymer a darker color. Unfortunately, this use has not been widely employed since the background goes up substantially, and little improvement in signal-to-noise ratio is generally found. Similarly osmium tetroxide can be added to the DAB polymer after it is formed. The osmium tetroxide reacts with the DAB product and leads to incorporation of the heavy metal, making the DAB deposit more visible in the electron microscope.